Hi. I too, am sorry to hear about your uncle. This is a mean disease.
I will echo what someone else said. It could be one of many things, or a combination of a couple or all of those things. Depression, meds, simply his way of dealing with it. The steroids can mess with him (anger). Just the diagnosis of cancer and going thru all of this can be simply depressing. We don't know if it was the tumors or the higher dose steroids for my mom, but when that medication went up, she was negative about a lot of stuff. She cursed a LOT more. However, she was never negative about the cancer or life itself.
My best advice? Never give up the fight. Ever. Never stop believing in your uncle. However, let him act how he wants. He's the one going thru this. He has the right to be depressed. Let him be obsessive about what ifs. Turn the table around and ask him to talk about it. Agree with him. Tell him yes, eventually one day, we all die. Ask if he wants to be an active part of his end of life planning. I found it easier to agree with mom when she was in her moods than not. Right or wrong, it didn't matter. I just told myself it was the medication and not my mother talking.
Also, what we did for mom: At her docs appt, one of us left the room with the doc and let him know how she had been acting. She would never admit to the doc she was depressed. She would put on a smile and tell him everything was perfect. Told the doc how she had been acting, her moods, etc. He told her that he was prescribing an antidepressant to counteract the effects of the steroids.
Sorry, I don't know anything about the clinical trial/vaccine.
Best wishes are with you and your family